Detect already merged videos

Without the '--keep-video' option the two files would be downloaded again and even using the option, ffmpeg would be run again, which for some videos can take a long time.
We use a temporary file with ffmpeg so that the final file only exists if it success

youtube_dl/YoutubeDL.py

@@ -1369,6 +1369,11 @@ class YoutubeDL(object):
                                             ' The formats won\'t be merged')
                     else:
                         postprocessors = [merger]
+                    if os.path.exists(encodeFilename(filename)):
+                        self.to_screen(
+                            '[download] %s has already been downloaded and '
+                            'merged' % filename)
+                    else:
                         for f in info_dict['requested_formats']:
                             new_info = dict(info_dict)
                             new_info.update(f)

youtube_dl/postprocessor/ffmpeg.py

@@ -580,9 +580,11 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
 class FFmpegMergerPP(FFmpegPostProcessor):
     def run(self, info):
         filename = info['filepath']
+        temp_filename = prepend_extension(filename, 'temp')
         args = ['-c', 'copy', '-map', '0:v:0', '-map', '1:a:0']
         self._downloader.to_screen('[ffmpeg] Merging formats into "%s"' % filename)
-        self.run_ffmpeg_multiple_files(info['__files_to_merge'], filename, args)
+        self.run_ffmpeg_multiple_files(info['__files_to_merge'], temp_filename, args)
+        os.rename(encodeFilename(temp_filename), encodeFilename(filename))
         return info['__files_to_merge'], info